The C18 compiler works fine after 60 days. You just lose high level optimizations, other than this there is no difference. (you are unlikely to use higher level optimizations). You can also use the free student edition to develope commercial applications with no other licenses.

I dont want to sound argumentative but from my opinion the PIC16F range are outdated. They were never designed for c programming and have a reduced instruction set (they were produced at the time when assembly was the leading embedded language). The PIC18 range is basically the same as the PIC16 range apart from an expanded instruction set which makes them easier to work with for c compilers. (Im pretty sure that any PIC16 has a pin to pin and language compatable PIC18).

Getting a book is probably the best thing you can do though.... Its not fully relevant but I do some java programming work occasionally (when its going ) and i can tell you that even though ive been into java since its first launch, id be lost without my reference manual!